Image display systems create images for human viewers to experience. The goal of such a display system is to simulate the experience of being at the location being displayed. The locations may be real, for example when a scene is recorded using a camera, imaginary, for example when a computer generates a scene using a database of shape and texture information, or a combination of real images and superimposed computer-generated images.
Regardless of the source of a particular image, the display system must be able to recreate complex color tones and intensities in order to make the recorded image appear life-like. To do this, the color spectrum of the display system must be correlated to the color spectrum of the device used to capture the image. This can be a particular challenge when displaying an image initially recorded on a continuous color media such as cinemagraphic film for display on a primary color based system such as a Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), Digital Micromirror Display (“DMD”)-based display, or plasma display. For the purposes of this application, the term “continuous color” used in conjunction with terms such as image, media, display, or system will refer to the characteristic of being comprised of a continuous spectrum of light compared to the term “primary color” which will refer to the characteristic of being comprised of light from discrete primary color bands. Primary color-based image display systems, such as the ones listed above, use light sources that create a limited color space or color gamut, which can be defined by a chromaticity diagram, as is further discussed below. Commonly, a standard CRT color gamut is the benchmark for RGB signals, where positive RGB signals will define the color space that is formed by a CRT monitor according to the self-luminance properties of the phosphors used.
The perceived color of an object is determined by the wavelength of the light emitted by or reflected by the object. The human eye contains sensors, called rods and cones, that detect the light from the object focused on the retina. Rods are responsible for low light vision. Cones are responsible for color vision. There are three types of cones in the human eye, each with a distinct wavelength passband. Using outputs from the three types of cones, the human brain creates the perception of color and intensity for each portion of an image.
Continuous color media recreate the original image spectrum for each portion of the image. In the case of photographic film, this is accomplished by absorbing the unwanted portions of the spectrum of light from a source while reflecting or transmitting the portions needed to create an image. Primary color systems have a limited spectrum, or color gamut, and therefore cannot recreate the entire spectrum of the original image, but instead create the perception of the original image by stimulating the three types of cones to produce a response that approximates that would have been produced by the original spectrum. Thus, three carefully chosen light sources (red, green, and blue) can be used to provide the perception of a continuous color spectrum.
The three colors chosen to be the primary colors of a primary color display system determine the available color space of the display system. Light sources from the primary color systems have characteristics that narrow the systems' effective color gamut or color space. CRT and plasma displays will have a specific color gamut based on the light emission spectrum of the phosphors they contain, while other projection systems may have a color gamut that is defined by a filtered white light source. While a given set of primary colors may provide a very broad color space, the use of filters to select the given set of primary colors from a white light source often limits the maximum intensity the display system is capable of producing to less than a minimum acceptable amount. Further, a given selection of color filters may result in a white level, formed by combining the three primary colors, that has an undesirable color tint.
While an ideal display can create a high intensity display of very pure colors including white, real world display systems must make tradeoffs among the white level, purity of the primary colors, and the maximum available brightness. These tradeoffs further affect the secondary colors because the secondary colors are formed by combining primary colors at intensities that are set relative to the maximum intensities of those primary colors. Thus, once the primary color filters are selected, the white point and the purity of the secondary colors are also determined.